With the increasing scale of integration of semiconductor circuits, as well as the advances in the technology of ultrahigh precision working, it has become necessary to keep surveillance of the fabrication process, particularly for the oxygen concentration of the process gas which need be monitored over the range from the ppb to sub-ppb level. In such precision measurements, even the oxygen analyzers that have heretofore been used without any problems are found to cause various problems and need be improved.
Among the problems that need be dealt with are that of back flow due to the pressure difference between a pipe for entrance gas to the detector and a pipe for exit gas, the problem of back diffusion of aerial oxygen from the pipe exit gas, the problem due to the dead zones in the apparatus and, further, the problem of dissolved oxygen in the water to be additionally supplied to the detector. As typified by these problems, it has been found that the results of measurements are affected by the presence of such small amounts of oxygen as have been insignificant to the conventional oxygen analyzers.
The present invention has been accomplished with a view to meeting these rigorous requirements. In order to enable precise measurements, the present inventors searched for certain factors of variations ascribable to the analyzer, located the causes of such variations and have proposed effective countermeasures. Thus, the problems to be solved by the invention are novel ones that have been first identified by the present inventors. It should be noted that the term "very small amounts" as used with oxygen in the specification refers to concentrations of no more than about 1 ppm.